Gas sensor and nitrogen oxide sensor

ABSTRACT

Disclosed are a gas sensor and a nitrogen oxide sensor each comprising a first diffusion rate-determining section for giving a predetermined diffusion resistance to a measurement gas to be introduced into a first chamber, the first diffusion rate-determining section being formed with a slit having a laterally extending aperture formed at a front end portion of a second spacer layer to make contact with the lower surface of a second solid electrolyte layer in which the aperture is formed to extend with an identical aperture width up to the first chamber, and a slit having a laterally extending aperture formed at a front end portion of the second spacer layer to make contact with the upper surface of a first solid electrolyte layer in which the aperture is formed to extend with an identical aperture width up to the first chamber. The slits have a substantially identical cross-sectional configuration, in which the length in the vertical direction is not more than 10 μm, and the length in the lateral direction is about 2 mm. Accordingly, it is possible to avoid the influence of the pulsation of the exhaust gas pressure generated in the measurement gas, and it is possible to improve the measurement accuracy obtained on the detecting electrode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a gas sensor and a nitrogen oxidesensor for measuring oxides such as O₂, NO, NO₂, SO₂, CO₂, and H₂Ocontained in, for example, atmospheric air and exhaust gas dischargedfrom vehicles or automobiles, and inflammable gases such as CO and CnHm.

2. Description of the Related Art

Those hitherto known as the method for measuring NOx in a measurementgas such as combustion gas include a technique in which the NOx-reducingability of Rh is utilized to use a sensor comprising a Pt electrode andan Rh electrode formed on an oxygen ion-conductive solid electrolytesuch as zirconia so that an electromotive force generated between theboth electrodes is measured.

However, the sensor as described above suffers the following problems.That is, the electromotive force is greatly changed depending on thechange in concentration of oxygen contained in the combustion gas as themeasurement gas. Moreover, the change in electromotive force is smallwith respect to the change in concentration of NOx. For this reason, theconventional sensor tends to suffer influence of noise.

Further, in order to bring out the NOx-reducing ability, it isindispensable to use a reducing gas such as CO. For this reason, theamount of produced CO is generally smaller than the amount of producedNOx under a lean fuel combustion condition in which a large amount ofNOx is produced. Therefore, the conventional sensor has a drawback inthat it is impossible to perform the measurement for a combustion gasproduced under such a combustion condition.

In order to solve the problems as described above, for example, JapaneseLaid-Open Patent Publication No. 8-271476 discloses a NOx sensorcomprising pumping electrodes having different NOx-decomposing abilitiesarranged in a first internal space which communicates with a measurementgas-existing space and in a second internal space which communicateswith the first internal space, and a method for measuring the NOxconcentration in which the O₂ concentration is adjusted by using a firstpumping cell arranged in the first internal space, and NO is decomposedby using a decomposing pump arranged in the second internal space sothat the NOx concentration is measured on the basis of a pumping currentflowing through the decomposing pump.

Further, Japanese Laid-Open Patent Publication No. 9-113484 discloses asensor element comprising an auxiliary pumping electrode arranged in asecond internal space so that the oxygen concentration in the secondinternal space is controlled to be constant even when the oxygenconcentration is suddenly changed.

The following fact has been revealed when a gas sensor is attached to anexhaust system of an internal combustion engine such as an automobileengine, and the internal combustion engine is operated. That is, inordinary cases, the sensor output ordinarily makes proportional changebased on an anchoring point of zero in accordance with the change inoxygen concentration as shown by a solid line “a” in FIG. 32. However,under a specified operation condition, the sensor output is subjected toshift-up as a whole as shown by a solid line “b”.

In general, as shown in FIG. 33, the total pressure of the exhaust gasdischarged from the automobile engine is composed of a constant staticpressure and a dynamic pressure generated by the pulsation of theexhaust gas pressure. The fluctuation cycle of the dynamic pressure issynchronized with the explosion cycle of the engine. As a result ofinvestigation on the cause of the shift-up of the sensor output, it hasbeen revealed that the shift-up occurs when the pulsation amount(=dynamic pressure) of the exhaust gas pressure is large as comparedwith the static pressure.

That is, as shown in FIG. 34, the shift amount of the sensor output hasbeen measured with respect to the ratio between the dynamic pressure andthe static pressure (dynamic pressure/static pressure). As a result, theshift amount is approximately zero when the dynamic pressure/staticpressure is not more than about 25%. However, the shift amount increasesproportionally from the stage at which the dynamic pressure/staticpressure exceeds about 25%.

Therefore, when the dynamic pressure is increased, it is inevitable tosuffer any deterioration of the correlation between the oxygen-pumpingamount effected by the main pump in the first space and the oxygenconcentration in the measurement gas. It is feared that the disturbanceof the oxygen concentration caused in the first space may bring aboutany deterioration concerning the control of the oxygen concentration inthe second space communicating with the first space and the accuracy ofmeasurement effected by the detecting electrode as the NOx-detectingsection.

SUMMARY OF THE INVENTION

The present invention has been made taking such problems intoconsideration, an object of which is to provide a gas sensor and anitrogen oxide sensor which make it possible to avoid the influence ofthe pulsation of the exhaust gas pressure generated in the measurementgas, and improve the measurement accuracy obtained on the detectingelectrode.

According to the present invention, there is provided a gas sensor formeasuring an amount of a measurement gas component contained in ameasurement gas existing in external space; the gas sensor at leastcomprising a substrate composed of a solid electrolyte to make contactwith the external space; an internal space formed at the inside of thesubstrate; a diffusion rate-determining means formed with a slit forintroducing the measurement gas from the external space via agas-introducing port under a predetermined diffusion resistance; and apumping means including an inner pumping electrode and an outer pumpingelectrode formed at the inside and outside of the internal spacerespectively, for pumping-processing oxygen contained in the measurementgas introduced from the external space, on the basis of a controlvoltage applied between the electrodes; wherein a dimension of a certainfactor for forming a cross-sectional configuration of the diffusionrate-determining means is not more than 10 μm.

The limiting current value Ip in the pumping means is approximated bythe following theoretical expression for the limiting current.

Ip≈(4F/RT)×D×(S/L)×(POe−POd)

In the expression, F represents the Faraday constant (=96500 A/sec), Rrepresents the gas constant (=82.05 cm³·atm/mol·K), T represents theabsolute temperature (K), D represents the diffusion coefficient(cm²/sec), S represents the cross-sectional area (cm²) of the diffusionrate-determining means, L represents the passage length (cm) of thediffusion rate-determining means, POe represents the partial pressure ofoxygen (atm) at the outside of the diffusion rate-determining means, andPOd represents the partial pressure of oxygen (atm) at the inside of thediffusion rate-determining means.

The present invention defines the factor for forming the cross-sectionalarea S of the diffusion rate-determining means in the theoreticallimiting current expression. Especially, it is defined that the certainfactor of the dimension for forming the cross-sectional area S is notmore than 10 μm.

In this arrangement, the pulsation (=dynamic pressure) of the exhaustgas pressure is attenuated by the wall resistance of the diffusionrate-determining means.

Specifically, the attenuation is effected up to the level at which theratio between the dynamic pressure and the static pressure (dynamicpressure/static pressure) is not more than 25%. Therefore, it ispossible to effectively suppress the shift-up phenomenon of the sensoroutput which would be otherwise caused by the fluctuation of the dynamicpressure.

It is also preferable for the gas sensor constructed as described abovethat when the cross-sectional configuration of the diffusionrate-determining means is formed with at least one lateral type slit,the certain factor is a length of the slit in a vertical direction.Alternatively, it is also preferable that when the cross-sectionalconfiguration of the diffusion rate-determining means is formed with atleast one vertical type slit, the certain factor is a length of the slitin a lateral direction.

It is also preferable for the gas sensor constructed as described abovethat a buffering space is provided between the gas-introducing port andthe diffusion rate-determining means. Usually, the oxygen suddenlyenters the sensor element via the gas-introducing port due to thepulsation of the exhaust gas pressure brought about in the externalspace. However, in this arrangement, the oxygen from the external spacedoes not enter the processing space directly, but it enters thebuffering space disposed at the upstream stage thereof. In other words,the sudden change in oxygen concentration, which is caused by thepulsation of the exhaust gas pressure, is counteracted by the bufferingspace. Thus, the influence of the pulsation of the exhaust gas pressure,which is exerted on the internal space, is in an almost negligibledegree.

As a result, the correlation is improved between the oxygen-pumpingamount effected by the pumping means in the processing space and theoxygen concentration in the measurement gas. It is possible to improvethe measurement accuracy obtained on the measuring pumping means or theconcentration-detecting means. Simultaneously, for example, it ispossible to concurrently use the internal space as a sensor fordetermining the air-fuel ratio.

It is also preferable for the gas sensor constructed as described abovethat a clogging-preventive section and the buffering space are providedin series between the gas-introducing port and the internal space(processing space); a front aperture of the clogging-preventive sectionis used to form the gas-introducing port; and a diffusionrate-determining section for giving a predetermined diffusion resistanceto the measurement gas is provided between the clogging-preventivesection and the buffering space.

In this arrangement, the gas sensor is prevented from clogging ofparticles (for example, soot and oil combustion waste) produced in themeasurement gas in the external space, which would be otherwise causedin the vicinity of the inlet of the buffering space. Thus, it ispossible to measure the predetermined gas component more accurately.Further, it is possible to maintain a highly accurate state over a longperiod of time.

It is also preferable for the gas sensor constructed as described abovethat the oxygen contained in the measurement gas introduced from theexternal space into the internal space is pumping-processed by using thepumping means to make control so that a partial pressure of oxygen inthe internal space (processing space) has a predetermined value at whicha predetermined gas component in the measurement gas is notdecomposable.

It is also preferable that the gas sensor further comprises a measuringpumping means for decomposing the predetermined gas component containedin the measurement gas after being pumping-processed by the pumpingmeans, by means of catalytic action and/or electrolysis, and pumpingprocessing oxygen produced by the decomposition; wherein thepredetermined gas component contained in the measurement gas is measuredon the basis of a pumping current flowing through the measuring pumpingmeans in accordance with the pumping process effected by the measuringpumping means.

Alternatively, it is also preferable that the gas sensor furthercomprises an oxygen partial pressure-detecting means for decomposing thepredetermined gas component contained in the measurement gas after beingpumping-processed by the pumping means, by means of catalytic action,and generating an electromotive force corresponding to a differencebetween an amount of oxygen produced by the decomposition and an amountof oxygen contained in a reference gas; wherein the predetermined gascomponent contained in the measurement gas is measured on the basis ofthe electromotive force detected by the oxygen partialpressure-detecting means.

According to another aspect of the present invention, there is provideda nitrogen oxide sensor for measuring an amount of a nitrogen oxidecomponent contained in a measurement gas existing in external space; thenitrogen oxide sensor at least comprising a substrate composed of anoxygen ion-conductive solid electrolyte to make contact with theexternal space; a first internal space formed at the inside of thesubstrate and communicating with the external space; a first diffusionrate-determining means formed with a slit for introducing themeasurement gas into the first internal space under a predetermineddiffusion resistance; a main pumping means including a first innerpumping electrode and a first outer pumping electrode formed at theinside and outside of the first internal space respectively, forpumping-processing oxygen contained in the measurement gas introducedfrom the external space, on the basis of a control voltage appliedbetween the electrodes so that a partial pressure of oxygen in the firstinternal space is controlled to have a predetermined value at which NOis not substantially decomposable; a second internal space communicatingwith the first internal space; a second diffusion rate-determining meansformed with a slit for introducing an atmosphere pumping-processed inthe first internal space into the second internal space under apredetermined diffusion resistance; and a measuring pumping meansincluding a second inner pumping electrode and a second outer pumpingelectrode formed at the inside and outside of the second internal spacerespectively, for decomposing NO contained in the atmosphere introducedfrom the first internal space, by means of catalytic action and/orelectrolysis to pumping-process oxygen produced by the decomposition;wherein the amount of nitrogen oxide contained in the measurement gas ismeasured on the basis of a pumping current flowing through the measuringpumping means in accordance with the pumping process effected by themeasuring pumping means; and a dimension of a certain factor for forminga cross-sectional configuration of at least one of the diffusionrate-determining means is not more than 10 μm

According to the present invention, the pulsation (=dynamic pressure) ofthe exhaust gas pressure is attenuated by the wall resistance of thediffusion rate-determining means. Specifically, the attenuation iseffected up to the level at which the ratio between the dynamic pressureand the static pressure (dynamic pressure/static pressure) is not morethan 25%. Therefore, it is possible to effectively suppress the shift-upphenomenon of the sensor output which would be otherwise caused by thefluctuation of the dynamic pressure.

According to still another aspect of the present invention, there isprovided a nitrogen oxide sensor for measuring an amount of a nitrogenoxide component contained in a measurement gas existing in externalspace; the nitrogen oxide sensor at least comprising a substratecomposed of an oxygen ion-conductive solid electrolyte to make contactwith the external space: a first internal space formed at the inside ofthe substrate and communicating with the external space; a firstdiffusion rate-determining means formed with a slit for introducing themeasurement gas into the first internal space under a predetermineddiffusion resistance; a main pumping means including an inner pumpingelectrode and an outer pumping electrode formed at the inside andoutside of the first internal space respectively, for pumping-processingoxygen contained in the measurement gas introduced from the externalspace, on the basis of a control voltage applied between the electrodesso that a partial pressure of oxygen in the first internal space iscontrolled to have a predetermined value at which NO is notsubstantially decomposable; a second internal space communicating withthe first internal space; a second diffusion rate-determining meansformed with a slit for introducing an atmosphere pumping-processed inthe first internal space into the second internal space under apredetermined diffusion resistance; and an oxygen partialpressure-detecting means including an inner measuring electrode and anouter measuring electrode formed at the inside and outside of the secondinternal space respectively, for decomposing NO contained in theatmosphere introduced from the first internal space, by means ofcatalytic action to generate an electromotive force corresponding to adifference between an amount of oxygen produced by the decomposition andan amount of oxygen contained in a reference gas; wherein the amount ofnitrogen oxide contained in the measurement gas is measured on the basisof the electromotive force detected by the oxygen partialpressure-detecting means; and a dimension of a certain factor forforming a cross-sectional configuration of at least one of the diffusionrate-determining means is not more than 10 μm.

Also in this aspect, the pulsation (=dynamic pressure) of the exhaustgas pressure is attenuated by the wall resistance of the diffusionrate-determining means. Specifically, the attenuation is effected up tothe level at which the ratio between the dynamic pressure and the staticpressure (dynamic pressure/static pressure) is not more than 25%.Therefore, it is possible to effectively suppress the shift-upphenomenon of the sensor output which would be otherwise caused by thefluctuation of the dynamic pressure.

The above and other objects, features, and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings in which a preferredembodiment of the present invention is shown by way of illustrativeexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a front view illustrating an arrangement of a gas sensoraccording to a first embodiment;

FIG. 1B shows a plan view thereof;

FIG. 2 shows a sectional view taken along a line II—II shown in FIG. 1B;

FIG. 3A shows a front view illustrating an arrangement concerningWorking Example used in first and second illustrative experiments;

FIG. 3B shows a plan view thereof;

FIG. 4 shows a perspective view illustrating the arrangement concerningWorking Example used in the first and second illustrative experiments,especially illustrating extracted arrangements of first and seconddiffusion rate-determining sections;

FIG. 5A shows a front view illustrating an arrangement concerningComparative Example used in the first and second illustrativeexperiments;

FIG. 5B shows a plan view thereof;

FIG. 6 shows a perspective view illustrating the arrangement concerningComparative Example used in the first and second illustrativeexperiments, especially illustrating extracted arrangements of first andsecond diffusion rate-determining sections;

FIG. 7 shows the characteristic illustrating the change in sensor outputwith respect to the oxygen concentration, obtained when the measurementcondition is changed in Comparative Example:

FIG. 8 shows the characteristic illustrating the change in sensor outputwith respect to the NOx concentration, obtained when the measurementcondition is changed in Comparative Example;

FIG. 9 shows the characteristic illustrating the change in sensor outputwith respect to the oxygen concentration, obtained when the measurementcondition is changed in Working Example;

FIG. 10 shows the characteristic illustrating the change in sensoroutput with respect to the NOx concentration, obtained when themeasurement condition is changed in Working Example;

FIG. 11A shows a waveform illustrating the fluctuation of the exhaustgas pressure in the vicinity of a gas-introducing port in ComparativeExample;

FIG. 11B shows a waveform illustrating the fluctuation of the exhaustgas pressure in the vicinity of an inlet of a first chamber;

FIG. 12A shows a waveform illustrating the fluctuation of the exhaustgas pressure in the vicinity of a gas-introducing port in WorkingExample;

FIG. 12B shows a waveform illustrating the fluctuation of the exhaustgas pressure in the vicinity of an inlet of a first chamber;

FIG. 13A shows a front view illustrating an arrangement of a gas sensoraccording to a first modified embodiment;

FIG. 13B shows a front view thereof;

FIG. 14 shows a sectional view taken along a line XIV—XIV shown in FIG.13B;

FIG. 15A shows a front view illustrating an arrangement of a gas sensoraccording to a second modified embodiment;

FIG. 15B shows a front view thereof;

FIG. 16 shows a sectional view taken along a line XVI—XVI shown in FIG.15B;

FIG. 17A shows a front view illustrating an arrangement of a gas sensoraccording to a third modified embodiment;

FIG. 17B shows a front view thereof;

FIG. 18 shows a sectional view taken along a line XVIII—XVIII shown inFIG. 17B;

FIG. 19A shows a front view illustrating an arrangement of a gas sensoraccording to a fourth modified embodiment;

FIG. 19B shows a front view thereof;

FIG. 20 shows a sectional view taken along a line XX—XX shown in FIG.19B;

FIG. 21A shows a front view illustrating an arrangement of a gas sensoraccording to a fifth modified embodiment;

FIG. 21B shows a front view thereof;

FIG. 22 shows a sectional view taken along a line XXII—XXII shown inFIG. 21B;

FIG. 23A shows a front view illustrating an arrangement of a gas sensoraccording to a sixth modified embodiment;

FIG. 23B shows a front view thereof;

FIG. 24 shows a sectional view taken along a line XXIV—XXIV shown inFIG. 23B;

FIG. 25A shows a front view illustrating an arrangement of a gas sensoraccording to a seventh modified embodiment;

FIG. 25B shows a front view thereof;

FIG. 26 shows a sectional view taken along a line XXVI—XXVI shown inFIG. 25B;

FIG. 27A shows a front view illustrating an arrangement of a gas sensoraccording to an eighth modified embodiment;

FIG. 27B shows a front view thereof;

FIG. 28 shows a sectional view taken along a line XXVIII—XXVIII shown inFIG. 27B;

FIG. 29A shows a front view illustrating an arrangement of a gas sensoraccording to a ninth modified embodiment;

FIG. 29B shows a front view thereof;

FIG. 30 shows a sectional view taken along a line XXX—XXX shown in FIG.29B;

FIG. 31 shows a sectional view illustrating an arrangement of a gassensor according to a second embodiment;

FIG. 32 shows the characteristic illustrating the change in sensoroutput with respect to the oxygen concentration concerning theconventional gas sensor;

FIG. 33 illustrates the total pressure of the exhaust gas dischargedfrom the automobile engine; and

FIG. 34 shows the characteristic illustrating the shift amount of thesensor output with respect to the ratio between the dynamic pressure andthe static pressure (dynamic pressure/static pressure).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Explanation will be made below with reference to FIGS. 1A to 31 forseveral illustrative embodiments in which the gas sensor according tothe present invention is applied to gas sensors for measuring oxidessuch as O₂, NO, NO₂, SO₂, CO₂, and H₂O contained in, for example,atmospheric air and exhaust gas discharged from vehicles or automobiles,and inflammable gases such as CO and CnHm.

As shown in FIGS. 1A, 1B, and 2, a gas sensor 10A according to the firstembodiment includes a sensor element 14 provided with a substratecomprising, for example, six stacked solid electrolyte layers 12 a to 12f composed of ceramics based on the use of oxygen ion-conductive solidelectrolytes such as ZrO₂.

In the sensor element 14, first and second layers from the bottom aredesignated as first and second substrate layers 12 a, 12 b respectively.Third and fifth layers from the bottom are designated as first andsecond spacer layers 12 c, 12 e respectively. Fourth and sixth layersfrom the bottom are designated as first and second solid electrolytelayers 12 d, 12 f respectively.

A space 16 (reference gas-introducing space 16), into which a referencegas such as atmospheric air to be used as a reference for measuringoxides is introduced, is formed between the second substrate layer 12 band the first solid electrolyte layer 12 d, the space 16 being compartedby a lower surface of the first solid electrolyte layer 12 d, an uppersurface of the second substrate layer 12 b, and side surfaces of thefirst spacer layer 12 c.

A first chamber 18 for adjusting the partial pressure of oxygen in ameasurement gas is formed and comparted between a lower surface of thesecond solid electrolyte layer 12 f and an upper surface of the firstsolid electrolyte layer 12 d. A second chamber 20 for finely adjustingthe partial pressure of oxygen in the measurement gas and measuringoxides, for example, nitrogen oxides (NOx) in the measurement gas isformed and comparted between the lower surface of the second solidelectrolyte layer 12 f and the upper surface of the first solidelectrolyte layer 12 d.

A gas-introducing port 22, which is formed at the forward end of thesensor element 14, communicates with the first chamber 18 via a firstdiffusion rate-determining section 26. The first chamber 18 communicateswith the second chamber 20 via a second diffusion rate-determiningsection 28.

In this embodiment, each of the first and second diffusionrate-determining sections 26, 28 gives a predetermined diffusionresistance to the measurement gas to be introduced into the firstchamber 18 and the second chamber 20 respectively. As shown in FIG. 1A,the first diffusion rate-determining section 26 is formed by twolaterally extending slits 30, 32. Specifically, the first diffusionrate-determining section 26 includes the slit 30 having a laterallyextending aperture formed at a front end portion of the second spacerlayer 12 e to make contact with the lower surface of the second solidelectrolyte layer 12 f, the aperture being formed to extend with anidentical aperture width up to the first chamber 18. The first diffusionrate-determining section 26 also includes the slit 32 having a laterallyextending aperture formed at a front end portion of the second spacerlayer 12 e to make contact with the upper surface of the first solidelectrolyte layer 12 d, the aperture being formed to extend with anidentical aperture width up to the first chamber 18.

In the first embodiment, the respective slits 30, 32 have approximatelythe same cross-sectional configuration. As shown in FIG. 1A, the lengthta in the vertical direction is not more than 10 μm. and the length tbin the lateral direction is about 2 mm.

The second diffusion rate-determining section 28 is also formed with twolaterally extending slits 34, 36 each having a cross-sectionalconfiguration similar to those of the first diffusion rate-determiningsection 26. It is allowable that a porous member composed of ZrO₂ or thelike is arranged and packed in each of the slits 34, 36 of the seconddiffusion rate-determining section 28 so that the diffusion resistanceof the second diffusion rate-determining section 28 is larger than thediffusion resistance of the first diffusion rate-determining section 26.It is preferable that the diffusion resistance of the second diffusionrate-determining section 28 is larger than that of the first diffusionrate-determining section 26. However, no problem occurs even when theformer is smaller than the latter.

The atmosphere in the first chamber 18 is introduced into the secondchamber 20 under the predetermined diffusion resistance via the seconddiffusion rate-determining section 28.

An inner pumping electrode 40 having a substantially rectangular planarconfiguration and composed of a porous cermet electrode (for example, acermet electrode of Pt·ZrO₂ containing 1% Au) is formed on the entirelower surface portion for forming the first chamber 18, of the lowersurface of the second solid electrolyte layer 12 f. An outer pumpingelectrode 42 is formed on a portion corresponding to the inner pumpingelectrode 40, of the upper surface of the second solid electrolyte layer12 f. An electrochemical pumping cell, i.e., a main pumping cell 44 isconstructed by the inner pumping electrode 40, the outer pumpingelectrode 42, and the second solid electrolyte layer 12 f interposedbetween the both electrodes 40, 42.

A desired control voltage (pumping voltage) Vp1 is applied between theinner pumping electrode 40 and the outer pumping electrode 42 of themain pumping cell 44 by the aid of an external variable power source 46to allow a pumping current Ip1 to flow in a positive or negativedirection between the outer pumping electrode 42 and the inner pumpingelectrode 40. Thus, the oxygen in the atmosphere in the first chamber 18can be pumped out to the external space, or the oxygen in the externalspace can be pumped into the first chamber 18.

A reference electrode 48 is formed on a lower surface portion exposed tothe reference gas-introducing space 16, of the lower surface of thefirst solid electrolyte layer 12 d. An electrochemical sensor cell,i.e., a controlling oxygen partial pressure-detecting cell 50 isconstructed by the inner pumping electrode 40, the reference electrode48, the second solid electrolyte layer 12 f, the second spacer layer 12e, and the first solid electrolyte layer 12 d.

The controlling oxygen partial pressure-detecting cell 50 is operated asfollows. That is, an electromotive force V1 is generated between theinner pumping electrode 40 and the reference electrode 48 on the basisof a difference in oxygen concentration between the atmosphere in thefirst chamber 18 and the reference gas (atmospheric air) in thereference gas-introducing space 16. The partial pressure of oxygen inthe atmosphere in the first chamber 18 can be detected by using theelectromotive force V1.

The detected value of the partial pressure of oxygen is used tofeedback-control the variable power source 46. Specifically, the pumpingoperation effected by the main pumping cell 44 is controlled by the aidof a feedback control system 52 for the main pump so that the partialpressure of oxygen in the atmosphere in the first chamber 18 has apredetermined value which is sufficiently low to control the partialpressure of oxygen in the second chamber 20 in the next step.

The feedback control system 52 comprises a circuit constructed tofeedback-control the pumping current Vp1 between the outer pumpingelectrode 42 and the inner pumping electrode 40 so that a difference(detection voltage V1) between an electric potential of the innerpumping electrode 40 and an electric potential of the referenceelectrode 48 is at a predetermined voltage level. In this embodiment,the inner pumping electrode 40 is grounded.

Therefore, the main pumping cell 44 pumps out or pumps in oxygen in anamount corresponding to the level of the pumping voltage Vp1, of themeasurement gas introduced into the first chamber 18. The oxygenconcentration in the first chamber 18 is subjected to feedback controlto give a predetermined level by repeating the series of operationsdescribed above. In this state, the pumping current Ip1, which flowsbetween the outer pumping electrode 42 and the inner pumping electrode40, indicates the difference between the oxygen concentration in themeasurement gas and the controlled oxygen concentration in the firstchamber 18. The pumping current Ip1 can be used to measure the oxygenconcentration in the measurement gas.

Each of the inner pumping electrode 40 and the outer pumping electrode42 is composed of a porous cermet electrode which is made of a metalsuch as Pt and ceramics such as ZrO₂. It is necessary to use a materialwhich has a weak reducing ability or no reducing ability with respect tothe NO component in the measurement gas, for the inner pumping electrode40 disposed in the first chamber 18 to make contact with the measurementgas. It is preferable that the inner pumping electrode 40 is composedof, for example, a compound having the perovskite structure such asLa₃CuO₄, a cermet comprising ceramics and a metal such as Au having alow catalytic activity, or a cermet comprising ceramics, a metal of thePt group, and a metal such as Au having a low catalytic activity. Whenan alloy composed of Au and a metal of the Pt group is used as anelectrode material, it is preferable to add Au in an amount of 0.03 to35% by volume of the entire metal component.

In the gas sensor 10A according to the first embodiment, a detectingelectrode 60 having a substantially rectangular planar configuration andcomposed of a porous cermet electrode is formed at a portion separatedfrom the second diffusion rate-determining section 28, on an uppersurface portion for forming the second chamber 20, of the upper surfaceof the first solid electrolyte layer 12 d. An alumina film forconstructing a third diffusion rate-determining section 62 is formed sothat the detecting electrode 60 is covered therewith. An electrochemicalpumping cell, i.e., a measuring pumping cell 64 is constructed by thedetecting electrode 60, the reference electrode 48, and the first solidelectrolyte layer 12 d.

The detecting electrode 60 is composed of a porous cermet comprisingzirconia as ceramics and a metal capable of reducing NOx as themeasurement gas component. Accordingly, the detecting electrode 60functions as a NOx-reducing catalyst for reducing NOx existing in theatmosphere in the second chamber 20. Further, the oxygen in theatmosphere in the second chamber 20 can be pumped out to the referencegas-introducing space 16 by applying a constant voltage Vp2 between thedetecting electrode 60 and the reference electrode 48 by the aid of a DCpower source 66. The pumping current Ip2, which is allowed to flow inaccordance with the pumping operation performed by the measuring pumpingcell 64, is detected by an ammeter 68.

The constant voltage (DC) power source 66 can apply a voltage of amagnitude to give a limiting current to the pumping for oxygen producedduring decomposition in the measuring pumping cell 64 under the inflowof NOx restricted by the third diffusion rate-determining section 62.

On the other hand, an auxiliary pumping electrode 70 having asubstantially rectangular planar configuration and composed of a porouscermet electrode (for example, a cermet electrode of Pt·ZrO₂ containing1% Au) is formed on the entire lower surface portion for forming thesecond chamber 20, of the lower surface of the second solid electrolytelayer 12 f. An auxiliary electrochemical pumping cell, i.e., anauxiliary pumping cell 72 is constructed by the auxiliary pumpingelectrode 70, the second solid electrolyte layer 12 f, the second spacerlayer 12 e, the first solid electrolyte layer 12 d, and the referenceelectrode 48.

The auxiliary pumping electrode 70 is based on the use of a materialhaving a weak reducing ability or no reducing ability with respect tothe NO component contained in the measurement gas, in the same manner asin the inner pumping electrode 40 of the main pumping cell 44. In thisembodiment, it is preferable that the auxiliary pumping electrode 70 iscomposed of, for example, a compound having the perovskite structuresuch as La₃CuO₄, a cermet comprising ceramics and a metal having a lowcatalytic activity such as Au, or a cermet comprising ceramics, a metalof the Pt group, and a metal having a low catalytic activity such as Au.Further, when an alloy comprising Au and a metal of the Pt group is usedas an electrode material, it is preferable to add Au in an amount of0.03 to 35% by volume of the entire metal components.

A desired constant voltage Vp3 is applied between the referenceelectrode 48 and the auxiliary pumping electrode 70 of the auxiliarypumping cell 72 by the aid of an external DC power source 74. Thus, theoxygen in the atmosphere in the second chamber 20 can be pumped out tothe reference gas-introducing space 16.

Accordingly, the partial pressure of oxygen in the atmosphere in thesecond chamber 20 is allowed to have a low value of partial pressure ofoxygen at which the measurement of the amount of the objective componentis not substantially affected, under the condition in which themeasurement gas component (NOx) is not substantially reduced ordecomposed. In this embodiment, owing to the operation of the mainpumping cell 44 for the first chamber 18, the change in amount of oxygenintroduced into the second chamber 20 is greatly reduced as comparedwith the change in the measurement gas. Accordingly, the partialpressure of oxygen in the second chamber 20 is accurately controlled tobe constant.

Therefore, in the gas sensor 10A according to the first embodimentconstructed as described above, the measurement gas, for which thepartial pressure of oxygen has been controlled in the second chamber 20,is introduced into the detecting electrode 60.

As shown in FIG. 2, the gas sensor 10A according to the first embodimentfurther comprises a heater 80 for generating heat in accordance withelectric power supply from the outside. The heater 80 is embedded in aform of being vertically interposed between the first and secondsubstrate layers 12 a, 12 b. The heater 80 is provided in order toincrease the conductivity of oxygen ion. An insulative layer 82 composedof alumina or the like is formed to cover upper and lower surfaces ofthe heater 80 so that the heater 80 is electrically insulated from thefirst and second substrate layers 12 a, 12 b.

The heater 80 is arranged over the entire portion ranging from the firstchamber 18 to the second chamber 20. Accordingly, each of the firstchamber 18 and the second chamber 20 is heated to a predeterminedtemperature. Simultaneously, each of the main pumping cell 44, thecontrolling oxygen partial pressure-detecting cell 50, and the measuringpumping cell 64 is also heated to a predetermined temperature andmaintained at that temperature.

Next, the operation of the gas sensor 10A according to the firstembodiment will be explained. At first, the forward end of the gassensor 10A is disposed in the external space. Accordingly, themeasurement gas is introduced into the first chamber 18 under thepredetermined diffusion resistance via the first diffusionrate-determining section 26 (slits 30, 32). The measurement gas, whichhas been introduced into the first chamber 18, is subjected to thepumping action for oxygen, caused by applying the predetermined pumpingvoltage Vp1 between the outer pumping electrode 42 and the inner pumpingelectrode 40 which construct the main pumping cell 44. The partialpressure of oxygen is controlled to have a predetermined value, forexample, 10⁻⁷ atm. The control is performed by the aid of the feedbackcontrol system 52.

The first diffusion rate-determining section 26 serves to limit theamount of diffusion and inflow of oxygen in the measurement gas into themeasuring space (first chamber 18) when the pumping voltage Vp1 isapplied to the main pumping cell 44 so that the current flowing throughthe main pumping cell 44 is suppressed.

In the first chamber 18, a state of partial pressure of oxygen isestablished, in which NOx in the atmosphere is not reduced by the innerpumping electrode 40 in an environment of being heated by the externalmeasurement gas and being heated by the heater 80. For example, acondition of partial pressure of oxygen is formed, in which the reactionof NO→1/2N₂+1/2O₂ does not occur, because of the following reason. Thatis, if NOx in the measurement gas (atmosphere) is reduced in the firstchamber 18, it is impossible to accurately measure NOx in the secondchamber 20 disposed at the downstream stage. In this context, it isnecessary to establish a condition in the first chamber 18 in which NOxis not reduced by the component which participates in reduction of NOx(in this case, the metal component of the inner pumping electrode 40).Specifically, as described above, such a condition is achieved by using,for the inner pumping electrode 40, the material having a low ability toreduce NOx, for example, an alloy of Au and Pt.

The gas in the first chamber 18 is introduced into the second chamber 20under the predetermined diffusion resistance via the second diffusionrate-determining section 28. The gas, which has been introduced into thesecond chamber 20, is subjected to the pumping action for oxygen, causedby applying the voltage Vp3 between the reference electrode 48 and theauxiliary pumping electrode 70 which constitute the auxiliary pumpingcell 72 to make fine adjustment so that the partial pressure of oxygenhas a constant and low value of partial pressure of oxygen.

The second diffusion rate-determining section 28 serves to limit theamount of diffusion and inflow of oxygen in the measurement gas into themeasuring space (second chamber 20) when the voltage Vp3 is applied tothe auxiliary pumping cell 72 so that the pumping current Ip3 flowingthrough the auxiliary pumping cell 72 is suppressed, in the same manneras performed by the first diffusion rate-determining section 26.

The measurement gas, which has been controlled for the partial pressureof oxygen in the second chamber 20 as described above, is introducedinto the detecting electrode 60 under the predetermined diffusionresistance via the third diffusion rate-determining section 62.

When it is intended to control the partial pressure of oxygen in theatmosphere in the first chamber 18 to have a low value of the partialpressure of oxygen which does not substantially affect the measurementof NOx, by operating the main pumping cell 44, in other words, when thepumping voltage Vp1 of the variable power source 46 is adjusted by theaid of the feedback control system 52 so that the voltage V1 detected bythe controlling oxygen partial pressure-detecting cell 50 is constant,if the oxygen concentration in the measurement gas greatly changes, forexample, in a range of 0 to 20%, then the respective partial pressuresof oxygen in the atmosphere in the second chamber 20 and in theatmosphere in the vicinity of the detecting electrode 60 slightly changein ordinary cases. This phenomenon is caused probably because of thefollowing reason. That is, when the oxygen concentration in themeasurement gas increases, the distribution of the oxygen concentrationoccurs in the widthwise direction and in the thickness direction in thefirst chamber 18. The distribution of the oxygen concentration changesdepending on the oxygen concentration in the measurement gas.

However, in the case of the gas sensor 10A according to the firstembodiment, the auxiliary pumping cell 72 is provided for the secondchamber 20 so that the partial pressure of oxygen in its internalatmosphere always has a constant low value of the partial pressure ofoxygen. Accordingly, even when the partial pressure of oxygen in theatmosphere introduced from the first chamber 18 into the second chamber20 changes depending on the oxygen concentration in the measurement gas,the partial pressure of oxygen in the atmosphere in the second chamber20 can be always made to have a constant low value, owing to the pumpingaction performed by the auxiliary pumping cell 72. As a result, thepartial pressure of oxygen can be controlled to have a low value atwhich the measurement of NOx is not substantially affected.

NOx in the measurement gas introduced into the detecting electrode 60 isreduced or decomposed around the detecting electrode 60. Thus, forexample, a reaction of NO→1/2N₂+1/2O₂ is allowed to occur. In thisprocess, a predetermined voltage Vp2, for example, 430 mV (700° C.) isapplied between the detecting electrode 60 and the reference electrode48 which construct the measuring pumping cell 64, in a direction to pumpout the oxygen from the second chamber 20 to the referencegas-introducing space 16.

Therefore, the pumping current Ip2 flowing through the measuring pumpingcell 64 has a value which is proportional to a sum of the oxygenconcentration in the atmosphere introduced into the second chamber 20,i.e., the oxygen concentration in the second chamber 20 and the oxygenconcentration produced by reduction or decomposition of NOx by the aidof the detecting electrode 60.

In this embodiment, the oxygen concentration in the atmosphere in thesecond chamber 20 is controlled to be constant by means of the auxiliarypumping cell 72. Accordingly, the pumping current Ip2 flowing throughthe pumping cell 64 is proportional to the NOX concentration. The NOxconcentration corresponds to the amount of diffusion of NOx limited bythe third diffusion rate-determining section 62. Therefore, even whenthe oxygen concentration in the measurement gas greatly changes, it ispossible to accurately measure the NOx concentration, based on the useof the measuring pumping cell 64 by the aid of the ammeter 68.

According to the fact described above, almost all of the pumping currentvalue Ip2 obtained by operating the measuring pumping cell 64 representsthe amount brought about by the reduction or decomposition of NOx.Accordingly, the obtained result does not depend on the oxygenconcentration in the measurement gas.

In the meantime, in ordinary cases, the shift amount increasesproportionally from the stage at which the ratio between the dynamicpressure and the static pressure (dynamic pressure/static pressure)exceeds about 25% (see FIG. 34). However, in the gas sensor 10Aaccording to the first embodiment, the length ta in the verticaldirection, which is the certain factor for forming the cross-sectionalconfiguration of the first diffusion rate-determining section 26 (slits30, 32), is not more than 10 μm.

The limiting current value Ip1 in the main pumping cell 44 isapproximated by the following theoretical expression for the limitingcurrent.

Ip1≈(4F/RT)×D×(S/L)×(POe−POd)

the expression, F represents the Faraday constant (=96500 A/sec), Rrepresents the gas constant (=82.05 cm³·atm/mol·K), T represents theabsolute temperature (K), D represents the diffusion coefficient(cm²/sec), S represents the cross-sectional area (cm²) of the firstdiffusion rate-determining section 26 (slit 30 or 32), L represents thepassage length (cm) of the first diffusion rate-determining section 26(slit 30 or 32), POe represents the partial pressure of oxygen (atm) inthe external space, and POd represents the partial pressure of oxygen(atm) in the first chamber 18.

In the gas sensor 10A according to the first embodiment, there isdefined the formation factor for the cross-sectional area S of the firstdiffusion rate-determining section 26 (slit 30 or 32) in the theoreticallimiting current expression. Especially, it is defined that the certainfactor of the dimension for forming the cross-sectional area S, i.e.,the length in the vertical direction in this embodiment is not more than10 μm.

Accordingly, the pulsation (=dynamic pressure) of the exhaust gaspressure is attenuated by the wall resistance of the first diffusionrate-determining section 26. Specifically, the attenuation is effectedup to the level at which the ratio between the dynamic pressure and thestatic pressure (dynamic pressure/static pressure) is not more than 25%.Therefore, it is possible to effectively suppress the shift-upphenomenon of the sensor output (the pumping current value Ip2concerning the measuring pumping cell or the value of the pumpingcurrent Ip1 flowing through the main pumping cell) which would beotherwise caused by the fluctuation of the dynamic pressure.

Two illustrative experiments (conveniently referred to as “first andsecond illustrative experiments”) will now be described. In the firstillustrative experiment, the measurement was made for the way of changeof the sensor output obtained when the oxygen concentration in themeasurement gas was changed concerning Working Example and ComparativeExample. In the second illustrative experiment, the measurement was madefor the way of change of the sensor output obtained when the NOxconcentration in the measurement gas was changed.

The following measurement condition was adopted. That is, a dieselengine of 2.5 L was used as an engine, the number of revolution was 1000to 4000 rpm, and the engine load was 5 to 20 kgm. The number ofrevolution, the engine load, and the EGR opening degree wereappropriately changed to measure the fluctuation of the sensor outputobtained under the respective conditions as well.

Working Example is illustrative of the case in which the first diffusionrate-determining section 26 was constructed by upper and lower twolaterally extending slits 30, 32 (length in the lateral direction: 2mm×length in the vertical direction: not more than 10 μm) as shown inFIGS. 3A, 3B, and 4 in the same manner as in the gas sensor 10Aaccording to the first embodiment. Comparative Example is illustrativeof the case in which the first diffusion rate-determining section 26 wasconstructed by a single slit 100 (length in the lateral direction: 0.2mm×length in the vertical direction: 0.2 mm) as shown in FIGS. 5A, 5B,and 6 concerning the gas sensor 10A according to the first embodiment.

Experimental results obtained in the first and second illustrativeexperiments are shown in FIGS. 7 and 8 (Comparative Example) and inFIGS. 9 and 10 (Working Example). It is understood that in ComparativeExample as shown in FIGS. 7 and 8, the sensor output fluctuates bychanging the measurement condition, and the fluctuation of the sensoroutput is conspicuous especially when the engine load is high.

The above result is obtained probably because of the following reason.That is, as shown in FIGS. 11A and 11B to depict waveforms, thefluctuation of the exhaust gas pressure in the vicinity of thegas-introducing port is approximately the same as the fluctuation of theexhaust gas pressure in the vicinity of the inlet of the first chamber18, and the fluctuation of the exhaust gas pressure associated with thechange in measurement condition directly affects the sensor output.

On the other hand, in Working Example, the sensor output does notfluctuates as shown in FIGS. 9 and 10 even when the measurementcondition is changed. It is possible to obtain the sensor outputdepending on the change in oxygen concentration and NOx concentration ata high degree of accuracy. This is probably because of the followingreason. That is, as shown in FIGS. 12A and 12B to depict waveforms, thefluctuation of the exhaust gas pressure in the vicinity of thegas-introducing port is attenuated by the wall resistance of the firstdiffusion rate-determining section 26. Therefore, the fluctuation of theexhaust gas pressure in the vicinity of the inlet of the first chamber18 is smaller than the fluctuation of the exhaust gas pressure in thevicinity of the gas-introducing port.

As described above, the gas sensor 10A according to the first embodimentmakes it possible to avoid the influence of the pulsation of the exhaustgas pressure generated in the measurement gas. Thus, it is possible toimprove the measurement accuracy obtained on the measuring pumping cell64.

Next, explanation will be made with reference to FIGS. 13A to 30 forseveral modified embodiments of the gas sensor 10A according to thefirst embodiment, namely modified embodiments principally concerning theshape of the first diffusion rate-determining section 26 and the seconddiffusion rate-determining section 28. In FIGS. 13A to 30, the electriccircuit system is omitted from the illustration in order to avoidcomplicated drawings. Components or parts corresponding to those shownin FIG. 1 are designated by the same reference numerals, duplicateexplanation of which will be omitted.

At first, as shown in FIGS. 13A, 13B, and 14, a gas sensor 10Aaaccording to the first modified embodiment differs in that the first andsecond diffusion rate-determining sections 26, 28 are formed by singlelaterally extending slits 110, 112 respectively.

Specifically, the first diffusion rate-determining section 26 includesthe slit 110 having a laterally extending aperture formed at a front endportion of the second spacer layer 12 e to make contact with the uppersurface of the first solid electrolyte layer 12 d, the aperture beingformed to extend with an identical aperture width up to the firstchamber 18. The second diffusion rate-determining section 28 includesthe slit 112 having an aperture formed at a terminal end portion of thefirst chamber 18 of the second spacer layer 12 e to make contact withthe upper surface of the first solid electrolyte layer 12 d, theaperture being formed to extend with an identical aperture width up tothe second chamber 20. In the first modified embodiment, each of theslits 110, 112 has approximately the same cross-sectional configuration,in which the length ta in the vertical direction is not more than 10 μm,and the length tb in the lateral direction is about 2 mm.

Next, as shown in FIGS. 15A, 15B, and 16, a gas sensor 10Ab according tothe second modified embodiment differs in that the first and seconddiffusion rate-determining sections 26, 28 are formed by singlelaterally extending wedge-shaped slits 114, 116 respectively.

Specifically, the first diffusion rate-determining section 26 includesthe wedge-shaped slit 114 having a laterally extending aperture formedat a front end portion of the second spacer layer 12 e to make contactwith the upper surface of the first solid electrolyte layer 12 d, theaperture being formed to extend with an aperture width (width in thevertical direction) gradually enlarged toward the first chamber 18. Thesecond diffusion rate-determining section 28 includes the wedge-shapedslit 116 having a laterally extending aperture formed at a terminal endportion of the first chamber 18 of the second spacer layer 12 e to makecontact with the upper surface of the first solid electrolyte layer 12d, the aperture being formed to extend with an aperture width graduallyenlarged toward the second chamber 20.

In the second modified embodiment, the minimum aperture at the front endof each of the wedge-shaped slits 114, 116 has approximately the samecross-sectional configuration, in which the length ta in the verticaldirection is not more than 10 μm, and the length tb in the lateraldirection is about 2 mm.

Next, as shown in FIGS. 17A, 17B, and 18, a gas sensor 10Ac according tothe third modified embodiment differs in that the first diffusionrate-determining section 26 is formed by three laterally extending slits118 a, 118 b, 118 c which are disposed in parallel to one another, andthe second diffusion rate-determining section 28 is formed by a singlelaterally extending slit 120.

Specifically, the first diffusion rate-determining section 26 includesthe three slits 118 a, 118 b, 118 c having three laterally extendingapertures formed in parallel to one another at front end portions of thesecond spacer layer 12 e to make contact with the upper surface of thefirst solid electrolyte layer 12 d, each of the apertures being formedto extend with an identical aperture width up to the first chamber 18.The second diffusion rate-determining section 28 includes the singleslit 120 having a single laterally extending aperture formed at aterminal end portion of the first chamber 18 of the second spacer layer12 e to make contact with the upper surface of the first solidelectrolyte layer 12 d, the aperture being formed to extend with anidentical aperture width up to the second chamber 20. In the thirdmodified embodiment, each of the slits 118 a, 118 b, 118 c, 120 has thelength ta in the vertical direction which is not more than 10 μm.

Next, as shown in FIGS. 19A, 19B, and 20, a gas sensor 10Ad according tothe fourth modified embodiment differs in that a space section 122 and abuffering space 124 are provided in series between the gas-introducingport 22 and the first diffusion rate-determining section 26, a frontaperture of the space section 122 constitutes the gas-introducing port22, and a fourth diffusion rate-determining section 126 for giving apredetermined diffusion resistance to the measurement gas is providedbetween the space section 122 and the buffering space 124.

Each of the first diffusion rate-determining section 26 and the seconddiffusion rate-determining section 28 is formed by two laterallyextending slits 30, 32, 34, 36, in the same manner as in the gas sensor10A according to the first embodiment.

The fourth diffusion rate-determining section 126 includes a slit 128having a laterally extending aperture formed at a terminal end portionof the space section 122 of the second spacer layer 12 e to make contactwith the lower surface of the second solid electrolyte layer 12 f, theaperture being formed to extend with an identical aperture width up tothe buffering space 124. The fourth diffusion rate-determining section126 further includes a slit 130 having a laterally extending apertureformed at a terminal end portion of the space section 122 of the secondspacer layer 12 e to make contact with the upper surface of the firstsolid electrolyte layer 12 d, the aperture being formed to extend withan identical aperture width up to the buffering space 124.

Next, as shown in FIGS. 21A, 21B, and 22, a gas sensor 10Ae according tothe fifth modified embodiment differs in that each of the first andsecond diffusion rate-determining sections 26, 28 is formed by a singlevertically extending slit 132, 134.

Specifically, the first diffusion rate-determining section 26 includesthe slit 132 having a vertically extending aperture formed at a frontend portion of the second spacer layer 12 e approximately at the centerin the widthwise direction thereof, the aperture being formed to extendwith an identical aperture width up to the first chamber 18. The seconddiffusion rate-determining section 28 includes the slit 134 having avertically extending aperture formed at a terminal end portion of thefirst chamber 18 of the second spacer layer 12 e approximately at thecenter in the widthwise direction thereof, the aperture being formed toextend with an identical aperture width up to the second chamber 20. Inthe fifth modified embodiment, each of the slits 132, 134 hasapproximately the same cross-sectional configuration, in which thelength tc in the vertical direction is the same as the thickness of thesecond spacer layer 12 e, and the length td in the lateral direction isnot more than 10 μm.

Next, as shown in FIGS. 23A, 23B, and 24, a gas sensor 10Af according tothe sixth modified embodiment differs in that the first and seconddiffusion rate-determining sections 26, 28 are formed by singlevertically extending wedge-shaped slits 136, 138 respectively.

Specifically, the first diffusion rate-determining section 26 includesthe wedge-shaped slit 136 having a vertically extending aperture formedat a front end portion of the second spacer layer 12 e approximately atthe center in the widthwise direction thereof, the aperture being formedto extend with an aperture width (width in the lateral direction)gradually enlarged toward the first chamber 18. The second diffusionrate-determining section 28 includes the wedge-shaped slit 138 having avertically extending aperture formed at a terminal end portion of thefirst chamber 18 of the second spacer layer 12 e approximately at thecenter in the widthwise direction thereof, the aperture being formed toextend with an aperture width (width in the lateral direction) graduallyenlarged toward the second chamber 20.

In the sixth modified embodiment, the minimum aperture at the front endof each of the wedge-shaped slits 136, 138 has approximately the samecross-sectional configuration, in which the length tc in the verticaldirection is the same as the thickness of the second spacer layer 12 e,and the length td in the lateral direction is not more than 10 μm.

Next, as shown in FIGS. 25A, 25B, and 26, a gas sensor 10Ag according tothe seventh modified embodiment differs in that the first diffusionrate-determining section 26 is formed by a slit 140 having asubstantially hourglass-shaped planar configuration, and the seconddiffusion rate-determining section 28 is formed by a single verticallyextending slit 142.

Specifically, the first diffusion rate-determining section 26 includesthe substantially hourglass-shaped slit 140 starting from a laterallyextending aperture formed at a front end portion of the second spacerlayer 12 e, the aperture having its aperture width (width in the lateraldirection) gradually decreasing toward the approximate center of thefirst diffusion rate-determining section 26 in the depth direction toform a vertically extending slit 144, in which the aperture width (widthin the lateral direction) of the slit 144 gradually increases toward thefirst chamber 18 to form the substantially hourglass-shaped slit 140.

On the other hand, the second diffusion rate-determining section 28includes the slit 142 having a vertically extending aperture formed at aterminal end portion of the first chamber 18 of the second spacer layer12 e approximately at the center in the widthwise direction thereof, theaperture being formed to extend with an is identical aperture width upto the second chamber 20.

In the seventh modified embodiment, the minimum aperture (slit 144) ofthe hourglass-shaped slit 140 for constructing the first diffusionrate-determining section 26 has approximately the same cross-sectionalconfiguration as that of the slit 142 for constructing the seconddiffusion rate-determining section 28, in which the length tc in thevertical direction is the same as the thickness of the second spacerlayer 12 e, and the length td in the lateral direction is not more than10 μm.

Next, as shown in FIGS. 27A, 27B, and 28, a gas sensor 10Ah according tothe eighth modified embodiment differs in that the first diffusionrate-determining section 26 is formed by five vertically extending slits146 a to 146 e which are disposed in parallel to one another, and thesecond diffusion rate-determining section 28 is formed by a singlevertically extending slit 148.

Specifically, the first diffusion rate-determining section 26 includesthe five slits 146 a to 146 e having five vertically extending aperturesformed in parallel to one another at terminal end portions of the secondspacer layer 12 e, each of the apertures being formed to extend with anidentical aperture width up to the first chamber 18. The seconddiffusion rate-determining section 28 includes the single slit 148having a vertically extending aperture formed at a terminal end portionof the first chamber 18 of the second spacer layer 12 e approximately atthe center in the widthwise direction thereof, the aperture being formedto extend with an identical aperture width up to the second chamber 20.In the eighth modified embodiment, the length td in the lateraldirection of each of the slits 146 a to 146 e, 148 is not more than 10μm.

Next, as shown in FIGS. 29A, 29B, and 30, a gas sensor 10Ai according tothe ninth modified embodiment differs in that a space section 122 and abuffering space 124 are provided in series between the gas-introducingport 22 and the first diffusion rate-determining section 26, a frontaperture of the space section 122 constitutes the gas-introducing port22, and a fourth diffusion rate-determining section 126 for giving apredetermined diffusion resistance to the measurement gas is providedbetween the space section 122 and the buffering space 124.

Each of the first diffusion rate-determining section 26 and the seconddiffusion rate-determining section 28 is formed by a single laterallyextending slit 132, 134, in the same manner as in the gas sensor 10Aeaccording to the fifth modified embodiment (see FIGS. 21A, 21B, and 22).

The fourth diffusion rate-determining section 126 includes a slit 150having a vertically extending aperture formed at a terminal end portionof the space section 122 of the second spacer layer 12 e approximatelyat the center in the widthwise direction thereof, the aperture beingformed to extend with an identical aperture width up to the bufferingspace 124.

The gas sensors 10Aa to 10Ai according to the first to ninth modifiedembodiments make it possible to avoid the influence of the pulsation ofthe exhaust gas pressure generated in the measurement gas, in the samemanner as the gas sensor 10A according to the first embodiment. Thus, itis possible to improve the measurement accuracy obtained on themeasuring pumping cell 64.

Especially, the gas sensors 10Ad and 10Ai according to the fourth andninth modified embodiments include the buffering space 124 provided atthe upstream stage of the first diffusion rate-determining section 26.Usually, the oxygen suddenly enters the sensor element 14 via thegas-introducing port 22 due to the pulsation of the exhaust gas pressurein the external space. However, in this arrangement, the oxygen from theexternal space does not directly enter the processing space, but itenters the buffering space 124 disposed at the upstream stage thereof.In other words, the sudden change in oxygen concentration, which iscaused by the pulsation of the exhaust gas pressure, is counteracted bythe buffering space 124. Thus, the influence of the pulsation of theexhaust gas pressure on the first chamber 18 is in an almost negligibledegree.

As a result, the correlation is improved between the oxygen-pumpingamount effected by the main pumping cell 44 for the first chamber 18 andthe oxygen concentration in the measurement gas. It is possible toimprove the measurement accuracy obtained by the measuring pumping cell64. Simultaneously, for example, it is possible to concurrently use thefirst chamber 18 as a sensor for determining the air-fuel ratio.

The gas sensors 10Ad and 10Ai according to the fourth and ninth modifiedembodiment described above include the space section 122 and thebuffering space 124 which are provided in series between thegas-introducing port 22 and the first diffusion rate-determining section26, and the front aperture of the space section 122 is used to form thegas-introducing port 22. The space section 122 functions as theclogging-preventive section for avoiding the clogging of particles (forexample, soot and oil combustion waste) produced in the measurement gasin the external space, which would be otherwise caused in the vicinityof the inlet of the buffering space 124. Accordingly, it is possible tomeasure the NOx component more accurately by using the measuring pumpingcell 64. Further, it is possible to maintain a highly accurate stateover a long period of time.

The gas sensor 10A according to the first embodiment and the gas sensors10Aa to 10Ad according to the first to fourth modified embodimentsdescribed above have their first and second diffusion rate-determiningsections 26, 28 each of which is constructed by the laterally extendingslit. The gas sensors 10Ae to 10Ai according to the fifth to ninthmodified embodiments have their first and second diffusionrate-determining sections 26, 28 each of which is constructed by thevertically extending slit. However, for example, the first diffusionrate-determining section 26 may be constructed by a laterally extendingslit, and the second diffusion rate-determining section 28 may beconstructed by a vertically extending slit. Alternatively, it is alsoallowable to adopt an arrangement in which the first and seconddiffusion rate-determining sections 26, 28 are constructed in an inversemanner of the above.

It is also allowable that the shape of each of the first and seconddiffusion rate-determining sections 26, 28 is not the slit-shapedconfiguration provided that the certain factor for constructing thecross-sectional area is not more than 10 μm. For example, an equivalenteffect can be obtained such that a sublimable fiber is embedded, and acylindrical diffusion rate-determining section having a diameter of notmore than 10 μm is constructed after the sintering.

Next, a gas sensor 10B according to the second embodiment will beexplained with reference to FIG. 31. Components or parts correspondingto those shown in FIG. 2 are designated by the same reference numerals,duplicate explanation of which will be omitted.

As shown in FIG. 31, the gas sensor 10B according to the secondembodiment is constructed in approximately the same manner as the gassensor 10A according to the first embodiment (see FIG. 2). However, theformer is different from the latter in that a measuring oxygen partialpressure-detecting cell 160 is provided in place of the measuringpumping cell 64.

The measuring oxygen partial pressure-detecting cell 160 comprises adetecting electrode 162 formed on an upper surface portion for formingthe second chamber 20, of the upper surface of the first solidelectrolyte layer 12 d, the reference electrode 48 formed on the lowersurface of the first solid electrolyte layer 12 d, and the first solidelectrolyte layer 12 d interposed between the both electrodes 162, 48.

In this embodiment, an electromotive force (electromotive force of anoxygen concentration cell) V2 corresponding to the difference in oxygenconcentration between the atmosphere around the detecting electrode 162and the atmosphere around the reference electrode 48 is generatedbetween the reference electrode 48 and the detecting electrode 162 ofthe measuring oxygen partial pressure-detecting cell 160.

Therefore, the partial pressure of oxygen in the atmosphere around thedetecting electrode 162, in other words, the partial pressure of oxygendefined by oxygen produced by reduction or decomposition of themeasurement gas component (NOx) is detected as a voltage value V2 bymeasuring the electromotive force (voltage) V2 generated between thedetecting electrode 162 and the reference electrode 48 by using avoltmeter 164.

Also in the gas sensor 10B according to the second embodiment, thepulsation (=dynamic pressure) of the exhaust gas pressure is attenuatedby the wall resistance of the first diffusion rate-determining section26. Therefore, it is possible to effectively suppress the shift-upphenomenon of the sensor output (pumping current value obtained by usingthe measuring pumping cell) which would be otherwise caused by thefluctuation of the dynamic pressure. As a result, it is possible toavoid the influence of the pulsation of the exhaust gas pressuregenerated in the measurement gas. It is possible to improve themeasurement accuracy concerning the measuring oxygen partialpressure-detecting cell 160.

The arrangements of the gas sensors 10Aa to 10Ai according to the firstto ninth modified embodiments can be also adopted to the gas sensor 10Baccording to the second embodiment.

The gas sensors 10A and 10B according to the first and secondembodiments described above (including the respective modifiedembodiments) are directed to oxygen and NOx as the measurement gascomponent to be measured. However, the present invention is alsoeffectively applicable to the measurement of bound oxygen-containing gascomponents such as H₂O and CO₂ other than NOx, in which the measurementis affected by oxygen existing in the measurement gas.

For example, the present invention is also applicable to gas sensorswhich are constructed to pump out O₂ produced by electrolysis of CO₂ orH₂O by using the oxygen pump, and to gas sensors in which H₂ produced byelectrolysis of H₂O is pumping-processed by using a protonion-conductive solid electrolyte.

It is a matter of course that the gas sensor and the nitrogen oxidesensor according to the present invention are not limited to theembodiments described above, which may be embodied in other variousforms without deviating from the gist or essential characteristics ofthe present invention.

As described above, according to the gas sensor and the nitrogen oxidesensor concerning the present invention, it is possible to avoid theinfluence of the pulsation of the exhaust gas pressure generated in themeasurement gas, and it is possible to improve the measurement accuracyobtained on the detecting electrode.

What is claimed is:
 1. A gas sensor for measuring an amount of ameasurement gas component contained in a measurement gas existing in anexternal space, said gas sensor at least comprising: a substratecomposed of a solid electrolyte to make contact with the external space;an internal space formed at the inside of said substrate; agas-introducing port for introducing measurement gas from the externalspace into said internal space; a diffusion rate-determining slitpassage for introducing measurement gas from the external space intosaid internal space via said gas-introducing port under a predetermineddiffusion resistance; a buffering space provided between saidgas-introducing port and said slit passage; and a pumping meansincluding an inner pumping electrode and an outer pumping electrodeformed at the inside and outside of said internal space respectively,for pumping-processing oxygen contained in the measurement gasintroduced from the external space, on the basis of a control voltageapplied between said electrodes, wherein: said slit passage has, whenviewed in a plane substantially perpendicular to a longitudinalextension axis thereof, two dimensions, and at least one dimension isnot more than 10 μm; and said buffering space has a cross-sectional areagreater than that of said slit passage.
 2. The gas sensor according toclaim 1, wherein: a clogging-preventative section and said bufferingspace are provided in series between said gas-introducing port and saidinternal space; a front aperture of said clogging-preventive section isused to form said gas-introducing port; and a diffusion rate-determiningsection for giving a predetermined diffusion resistance to saidmeasurement gas is provided between said clogging-preventive section andsaid buffering space.
 3. The gas sensor according to claim 1, whereinsaid oxygen contained in the measurement gas introduced from saidexternal space into the internal space is pumping-processed by usingsaid pumping means to make control so that a partial pressure of oxygenin said internal space has a predetermined value at which apredetermined gas component in the measurement gas is not decomposable.4. The gas sensor according to claim 3, further comprising: a measuringpumping means for decomposing said predetermined gas component containedin the measurement gas after being pumping-processed by said pumpingmeans, by means of catalytic action and/or electrolysis, and pumpingprocessing oxygen produced by said decomposition, wherein: thepredetermined gas component contained in said measurement gas ismeasured on the basis of a pumping current flowing through saidmeasuring pumping means in accordance with said pumping process effectedby said measuring pumping means.
 5. The gas sensor according to claim 3,further comprising: an oxygen partial pressure-detecting means fordecomposing said predetermined gas component contained in themeasurement gas after being pumping-processed by said pumping means, bymeans of catalytic action, and generating an electromotive forcecorresponding to a difference between an amount of oxygen produced bysaid decomposition and an amount of oxygen contained in a reference gas,wherein: the predetermined gas component contained in said measurementgas is measured on the basis of said electromotive force detected bysaid oxygen partial pressure-detecting means.
 6. The gas sensoraccording to claim 1, wherein said substrate is substantially planar andsaid at least one dimension of said slit passage extends generallyperpendicular to the plane of said substrate.
 7. The gas sensoraccording to claim 1, wherein said substrate is substantially planar andsaid at least one dimension of said slit passage extends generallywithin the plane of said substrate.
 8. A nitrogen oxide sensor formeasuring an amount of a nitrogen oxide component contained in ameasurement gas existing in an external space, said nitrogen oxidesensor at least comprising: a substrate composed of an oxygenion-conductive solid electrolyte to make contact with the externalspace; a first internal space formed at the inside of said substrate andcommunicating with the external space; a gas-introducing port forintroducing measurement gas from the external space into said firstinternal space; a first diffusion rate-determining slit passage forintroducing the measurement gas into said first internal space via saidgas-introducing port under a predetermined diffusion resistance; aclogging-preventive section provided between said gas-introducing portand said slit passage; and a main pumping means including a first innerpumping electrode and a first outer pumping electrode formed at theinside and outside of said first internal space respectively, forpumping-processing oxygen contained in the measurement gas introducedfrom said external space, on the basis of a control voltage appliedbetween said electrodes so that a partial pressure of oxygen in saidfirst internal space is controlled to have a predetermined value atwhich NO is not substantially decomposable; a second internal spacecommunicating with said first internal space; a second diffusionrate-determining slit passage for introducing an atmospherepumping-processed in said first internal space into said second internalspace under a predetermined diffusion resistance; and a measuringpumping means including a second inner pumping electrode and a secondouter pumping electrode formed at the inside and outside of said secondinternal space respectively, for decomposing NO contained in saidatmosphere introduced from said first internal space, by means ofcatalytic action and/or electrolysis to pumping-process oxygen producedby said decomposition, wherein: said amount of nitrogen oxide containedin the measurement gas is measured on the basis of a pumping currentflowing through said measuring pumping means in accordance with saidpumping process effected by said measuring pumping means; at least oneof said first and second slit passages has, when viewed in a planesubstantially perpendicular to longitudinal extension axes thereof, twodimensions, and at least one dimension is not more than 10 μm; and saidclogging-preventive section has a cross-sectional area greater than thatof said slit passage.
 9. A nitrogen oxide sensor for measuring an amountof a nitrogen oxide component contained in a measurement gas existing inan external space, said nitrogen oxide sensor at least comprising: asubstrate composed of an oxygen ion-conductive solid electrolyte to makecontact with the external space; a first internal space formed at theinside of said substrate and communicating with the external space; agas-introducing port for introducing measurement gas from the externalspace into said first internal space; a first diffusion rate-determiningslit passage for introducing the measurement gas into said firstinternal space via said gas-introducing port under a predetermineddiffusion resistance; a clogging-preventive section provided betweensaid gas-introducing port and said slit passage; and a main pumpingmeans including an inner pumping electrode and an outer pumpingelectrode formed at the inside and outside of said first internal spacerespectively, for pumping-processing oxygen contained in the measurementgas introduced from said external space, on the basis of a controlvoltage applied between said electrodes so that a partial pressure ofoxygen in said first internal space is controlled to have apredetermined value at which NO is not substantially decomposable; asecond internal space communicating with said first internal space; asecond diffusion rate-determining slit passage for introducing anatmosphere pumping-processed in said first internal space into saidsecond internal space under a predetermined diffusion resistance; and anoxygen partial pressure-detecting means including an inner measuringelectrode and an outer measuring electrode formed at the inside andoutside of said second internal space respectively, for decomposing NOcontained in said atmosphere introduced from said first internal space,by means of catalytic action to generate an electromotive forcecorresponding to a difference between an amount of oxygen produced bysaid decomposition and an amount of oxygen contained in a reference gas,wherein: said amount of nitrogen contained in the measurement gas ismeasured on the basis of said electromotive force detected by saidoxygen partial pressure-detecting means; at least one of said first andsecond slit passages has, when viewed in a plane substantiallyperpendicular to longitudinal extension axes thereof, two dimensions,and at least one dimension is not more than 10 μm; and saidclogging-preventive section has a cross-sectional area greater than thatof said slit passage.
 10. A gas sensor for measuring an amount of ameasurement gas component contained in a measurement gas existing in anexternal space, said gas sensor at least comprising: a substratecomposed of a solid electrolyte to make contact with the external space;an internal space formed at the inside of said substrate; agas-introducing port for allowing gas communication between the internaland external spaces; a clogging-preventive section positioned adjacentsaid gas-introducing port and having at least one dimensionsubstantially the same as that of said internal space; a diffusionrate-determining slit passage for introducing the measurement gas fromthe external space via the gas-introducing port under a predetermineddiffusion resistance; and a pumping means including an inner pumpingelectrode and an outer pumping electrode formed at the inside andoutside of said internal space respectively, for pumping processingoxygen contained in the measurement gas introduced from the externalspace, on the basis of a control voltage applied between saidelectrodes, wherein: said slit passage has, when viewed in a planesubstantially perpendicular to a longitudinal extension axis thereof,two dimensions, and at least one dimension is not more than 10 μm; andsaid clogging-preventive section has a cross-sectional area greater thanthat of said slit passage.